The composite image on the left is of the galaxy cluster Abell 85, located
about 740 million light years from Earth. The purple emission is
multi-million degree gas detected in X-rays by NASA's Chandra X-ray
Observatory and the other colors show galaxies in an optical image from the
Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by
Chandra to trace how dark energy has stifled the growth of these massive
structures over the last 7 billion years. Galaxy clusters are the largest
collapsed objects in the Universe and are ideal for studying the properties
of dark energy, the mysterious form of repulsive gravity that is driving
the accelerated expansion of the Universe.

The illustration on the right shows snapshots from a simulation by Volker
Springel, representing the growth of cosmic structure when the Universe was
0.9 billion, 3.2 billion and 13.7 billion years old (now). This shows how
the Universe has evolved from a smooth state to one containing a vast
amount of structure. Gas is shown in these snapshots, where the yellow
regions are stars and the brightest structures are galaxies and galaxy
clusters. The growth of these structures was initially driven only by the
attractive force of gravity, but then later there was competition with the
repulsive force of dark energy.

Understanding the nature of dark energy is one of the biggest problems in
science. Possibilities include the cosmological constant, equivalent to
the energy of empty space, a modification in general relativity on the
largest scales, or a more general physical field. To help decide between
these options, Chandra was used to study the increase in mass of galaxy
clusters with time over the last 7 billion years.
The results are remarkably consistent with those from previous results that
measure the expansion of the Universe using distance measurements,
revealing that general relativity works as expected on large scales. The
cluster work, in combination with other studies, also provides the
strongest evidence to date that dark energy is the cosmological constant,
or that `nothing weighs something'.

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